Such a storage facility is made by digging one or more caverns of appropriate shapes and sizes given the intended use and the nature of the terrain, or by modifying pre-existing natural or artificial voids such as mines, quarries, etc. The storage facility is located at a depth below the level of the water table in a water-saturated rock formation whose permeability depends on the site.
The pressure of the stored products is generally greater than atmospheric pressure and as a result it must be ensured that the cavern is impervious, i.e. that there is no possibility of the product penetrating into the ground by infiltration. Such imperviousness is generally obtained by means of a continuous flow of water into the cavern from the surrounding rock formation through the walls of the cavern. Under substantially constant pressure conditions, a permanent flow regime is established, and it has become common practice to suppose that the necessary height between the top of the cavern and the lowest point of the water table should not be less than the pressure of the gas in the cavern (expressed as a column of water), in order to ensure that the stored product does not infiltrate into the rock formation, i.e. in order to be sure that the product does not leak out, with the resulting pressure difference being considered as providing a margin of safety.
French published patent application No. 2 473 618 in the name of Geostock relates to a method of reducing the safety margin to as small a value as possible, and describes a magnitude called a "form parameter" which is expressed in pressure units and which makes it possible to determine the minimum safety margin which can be used.
However, these considerations are applicable solely to a permanent regime. when the pressure in the cavern varies, for example when its pressure is being raised to a "test" pressure in order to test its imperviousness, or when product is inserted therein at a temperature greater than its storage temperature, the flow regime around the cavern is no longer "permanent" and transient phenomena occur which may invert the direction of water flow and thus allow product to infiltrate to some extent into the rock formation around the cavern. The cavern is then no longer "impervious" in the sense described above. These transient phenomena then tend to die away progressively so that there is a return to the permanent regime. The duration of the transient phenomena depends, inter alia, on the nature and the permeability of the rock formation.
Thus, although the above-mentioned form parameter is a criterion representative of static imperviousness, a criterion is still required which is representative of dynamic imperviousness, i.e. of imperviousness under varying pressure conditions. The present invention relates to this problem.